Scientists have voted to change the definition of a kilogram after more than 100 years in a landmark decision.
Representatives from 60 countries voted in favour of measuring a kilogram by the Planck constant, ending decades of definition by a small piece of metal held in a vault in Paris.
Le Grand K, a small cylinder of titanium alloy, has set the standard since 1889. All the scales in the world are ultimately calibrated against it, even those weighing in pounds and ounces.
Although the mother of all kilograms has only been taken out of its protective case four times in the last century, it has lost atoms and therefore mass.
It amounts to just 20 billionths of a gram, about the weight of an eyelash, but those small changes are important to maintaining accuracy.
Martin Milton, director, International Bureau of Weights and Measures (BIPM) said: "The SI redefinition is a landmark moment in scientific progress.
"Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society's greatest challenges."
The changes will come into force on 20 May next year and also affect the ampere, the kelvin and the mole.
The ampere will now be defined by the elementary electrical charge, the kelvin by the Boltzmann constant and the mole by by the Avogadro constant.
Britain has a copy of Le Grand K called Kilo 18, which it won in a lottery in 1889, and is stored at the National Physical Laboratory (NPL) in west London.
Stuart Davidson, a metrologist or weight scientist at NPL, is one of the trusted guardians.
"Once you get up to a few tens of tonnes – things like filling an aircraft with fuel – everything needs to be traceable back to a standard," he told Sky News earlier this week.
"The same is true when you get down to very small masses like a milligram – for example the active ingredients of pharmaceuticals.
"You like to know you are getting the right dose of drugs when you are given a prescription."
Other important standard units have already been updated.
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